In the drilling of oil and gas wells, drilling fluids or "muds" are used to provide well bore lubrication, to cool the drill bit, to protect against corrosion and to provide a pressure head to maintain formation integrity. There are two main types of drilling muds: water-based and oil-based. Oil-based drilling muds are employed in operations where it is desirable to drill at elevated temperatures, improve bore hole stability, control shale sloughing, and control water wetting of the formation such as in clay and some shale formations. Oil-based drilling muds are also desirable in "sour gas" wells where the water in a water-based drilling mud can react with the formation sulfur compounds and cause hydrogen embrittlement of the steels employed in the drilling operations. Oil-based drilling muds also inhibit corrosion and provide superior lubrication of the drill pipe in the well bore such as, for example, during directional drilling operations often conducted from offshore platforms.
Drilling muds are typically circulated down the inside of a tubular drill string, outwardly through the drill bit and up the annulus between the drill string and the bore. Drilling muds serve to carry the drill cuttings away from the bit and out of the bore hole. A typical oil-based drilling mud includes a diesel or mineral oil base, oil soluble emulsifiers, water (often salt water), oil wetting agents such as calcium sulfonates and organic amines to provide for oil wetting of the solids, and additives to control leak-off into the formation such as gilsonite and organophilic clays. The density of the drilling mud is adjusted with weighting agents such as barite or hematite. Oil-based drilling muds are very stable oil external-water internal emulsions including wetting agents to hold solids such as drill cuttings in the oil phase. The drill cuttings thus tend to become oil wet, trapping large quantities of oil-based mud in their intergranular spaces and creating environmental concerns regarding disposal of the contaminated drill cuttings.
For example, uncleaned drill cuttings which are dumped offshore can cause substantial pollution as the oil is gradually released from surface adhesion to the drill cuttings. Because the industry and the U.S. government want to avoid any such oil pollution, regulations governing the disposal of drill cuttings or solids have been promulgated.
In the prior art, drill cuttings contaminated with oil-based drilling muds were often collected in settling tanks where re-usable drilling mud was drawn off the top of the tank and contaminated drill cuttings, as bottoms, were transported to appropriate disposal sites. Such storage and transportation operations are costly and environmentally undesirable especially in offshore drilling operations. Typically, oil contaminated cuttings contain about fifty percent (50%) by volume of oil-based liquid. The value of this large volume of entrained oily liquids is considerable, and there is a strong economic incentive to recover the oil-based drilling mud both for economic as well as environmental reasons. Further, even burying of such oil-covered drill cuttings at waste disposal sites on shore is undesirable since the oil may eventually leach off the surface of the drill solids and enter subsurface water supplies.
Several different methods for processing drill solids contaminated with oil-based mud have been attempted. For instance, U.S. Pat. Nos. 3,688,781; 3,693,733; 3,716,480; 4,175,039; 4,546,783 and 4,645,608 teach the use of aqueous solutions of detergents in an attempt to wash adsorbed oil-based mud from the surface of the drill cuttings. These methods have been largely unsuccessful because oil-based muds are specially formulated with powerful oil wetting agents that resist the detergent action of aqueous wash solutions. Additionally, detergent-laden water, which may be even more toxic to marine organisms than the oil on the drill solids, is continuously discharged into the marine environment.
U.S. Pat. Nos. 4,209,381 and 4,395,338 teach the use of steam to strip the more volatile oils from oily drill cuttings, followed in some cases by distillation of the remaining solids to remove the higher boiling oil fractions. The methods are impractical offshore because of the excessively high energy requirements to generate the quantity of steam needed and the high temperatures needed to distill the oil.
U.S. Pat. Nos. 4,139,462; 4,304,609; 4,411,074 and 4,606,283 all teach various thermal methods to heat the oil-laden solids to drive off the oil as a vapor. Typically, the high temperatures required for these processes is supplied by electrical resistance heating, electrical induction heating, infra-red heaters, or high temperature heat transfer fluids. The methods have been at least partly unsuccessful for reasons already cited. The total amount of energy to heat all of the solids and boil all of the liquids off the cuttings is excessively high. Also, it is very dangerous to operate any equipment offshore in which hydrocarbon vapors are generated at temperatures well above their flash point.
U.S. Pat. No. 4,040,866 teaches the use of a mutual solvent to clean oily drill cuttings. A mutual solvent is one that is soluble in both oil and water. In this process, oily liquid is removed from the solids with a mutual solvent like ethylene glycol monobutyl ether; however, the mutual solvent remains on the cuttings and must be washed away with water leaving the cuttings free of oil and solvent. This method has proven impractical because two undesirable process streams are created. Large quantities of solvent (approximately equal to the original volume of oily liquid on the solids) are washed from the solids with water and discharged with the water into the environment. It is probable that the solvent is even more toxic to marine organisms than the oil which was removed from the cuttings. Additionally, large volumes of mutual solvent become contaminated with dissolved oil and must be either discarded or purified and recycled. The cost of mutual solvents prohibits simple disposal. Further, the high boiling point and high latent heat of vaporization of mutual solvents make their separation from oil by distillation difficult, expensive and hazardous.
U.S. Pat. No. 4,434,028 teaches a high pressure process for the use of a solvent which is miscible with oil but essentially immiscible with water to clean oily drill cuttings. In this process, a substance that is typically a gas at ambient temperature and pressure is compressed sufficiently to convert the gas to a liquid which then becomes a suitable solvent for the oil associated with drill cuttings. The liquified gas is then flowed, batchwise, through a vessel packed with oily solids. When the solids have been washed sufficiently clean, the chamber is depressurized allowing the solvent to flash into a vapor, leaving the solids free of oil and solvent. The oil-contaminated solvent can also be flashed to a vapor to separate it from the oil and allow it to be recycled. This process has not been successful on offshore drill sites for several possible reasons. High pressure is required to convert the normally gaseous material to a liquid so it can dissolve the oil on the solids. Mechanical problems associated with moving solids repeatedly into and out of high pressure vessels without leakage are formidable. Also, the batchwise nature of the process is not compatible with the continuous process of drilling and generation of drill cuttings. Finally, mechanical crushing of the cuttings prior to extraction requires heavy, bulky, maintenance-prone equipment which is especially undesirable for uses in offshore drilling operations.